1. Field of the Invention
The present invention relates to an antenna capable of receiving circularly polarized waves and a semiconductor device having such an antenna.
2. Description of the Related Art
In recent years, RFID (radio frequency identification) systems have been researched and put into practical use.
RFID refers to a communication technology over electromagnetic waves between a reader/writer and a semiconductor device capable of wireless data transmission and reception (also called an RFID tag, ID tag, IC tag, IC chip, wireless tag, electronic tag, or wireless chip), so that data can be stored in or read from the semiconductor device. Such a semiconductor device has an antenna and an IC (integrated circuit) with a memory circuit, a signal processing circuit, and the like. As an antenna used for a semiconductor device, a dipole antenna, a folded dipole antenna, or the like is frequently used because of its simple structure.
Meanwhile, it is known that when a linear antenna such as a dipole antenna is provided in a semiconductor device and if the direction of polarized waves received by the linear antenna greatly differs from the direction of polarized waves transmitted from an antenna provided in a reader/writer, there arises a great loss of polarized waves. Therefore, considering the circumstance that such a semiconductor device is used while being attached to another object, the direction of attachment is undesirably restricted.
In order to solve the foregoing problem, an antenna capable of transmitting circularly polarized waves is often used as a transmission antenna of a reader/writer. Accordingly, communication can be performed regardless of the direction of polarized waves that can be received by a reception antenna. However, when a linear antenna receives electromagnetic waves from a transmission antenna that can transmit circularly polarized waves, there is a 3 dB loss of circularly polarized waves (e.g., Reference 1: Klaus Finkenzeller, RFID Handbook, 2nd Edition, The Nikkan Kogyo Shimbun, Ltd., May 2004, pp. 98-99). When there is such a loss of circularly polarized waves, electricity received by the semiconductor device decreases and, therefore, a communication distance between the reader/writer and the semiconductor device decreases. Thus, it is desired to reduce the loss of circularly polarized waves.
As an antenna suited to receiving circularly polarized waves, there are known antennas disclosed in Reference 2 (Japanese Published Patent Application No. H8-195617) and Reference 3 (Japanese Published Patent Application No. 2000-59241). These antennas are C-shaped loop elements having a dielectric substrate and a cut section and provided over the dielectric substrate. The C-shaped loop element is disposed so as to be opposite a ground plane with a predetermined interval therebetween.
Further, as a thin antenna capable of receiving circularly polarized waves, there has been developed an antenna which is a combination of a loop antenna and a parasitic element disposed outside the loop antenna (e.g., Reference 4: Japanese Published Patent Application No. 2005-102183).
For a typical antenna, a cable having an intrinsic impedance of 50Ω is used for feeding electricity, and a matching circuit is disposed between the cable and the antenna to perform impedance matching. When sufficient impedance matching is not performed, electricity received by the antenna is reflected by the input of the antenna, in which case electricity fed from the cable cannot be delivered to the antenna. As for a semiconductor device capable of wireless data transmission and reception, electricity is fed without using a cable or the like, but by directly connecting an antenna and an IC that constitute the semiconductor device. However, when there is an impedance mismatch between the antenna and the IC, electricity required to operate the IC may not be supplied from the antenna, in which case the semiconductor device cannot operate. Therefore, impedance matching between the antenna and the IC that constitute the semiconductor device is of great importance.
In addition, for a semiconductor device that wirelessly transmits and receives data, it is preferable to perform impedance matching without using a matching circuit in terms of cost saving and the like (e.g., Reference 5: Japanese Published Patent Application No. 2005-244283).